My invention relates to a multifrequency scanning radio receiver, and particularly to such a receiver that monitors a priority radio frequency while a nonpriority radio frequency signal is being received.
Scanning radio receivers are used to monitor a plurality of radio frequencies or channels. Often, one radio channel is designated the priority channel which should always be monitored for priority or important information. This priority monitoring has been achieved by a circuit or arrangement described in U.S. Pat. No. 3,619,788. In that arrangement, a scanning receiver is recurringly switched between radio channels until a signal is received, at which time the receiver is held on that channel for the duration of the signal. If the receiver is held on a nonpriority channel, a circuit is actuated to sample the priority channel repeatedly to determine whether a signal is present on the priority channel. If a priority channel signal is present during the sampling interval, the receiver is then held on the priority channel so that priority information will be heard, in spite of the fact that there was a signal on the nonpriority channel.
Such an arrangement works satisfactorily, particularly if crystal oscillators are used. Such oscillators permit rapid switching between channels in the order of ten milliseconds every 300-500 milliseconds. Such switching times do not cause significant distortion or deterioration in the speech quality of a signal being received. If a large number of channels must be monitored, it is more practical and economical to use a frequency synthesizer instead of a plurality of crystal oscillators. However, the limited bandwidth of the frequency synthesizer control circuit makes it quite difficult to achieve a settled synthesizer frequency to within two Kilohertz of the final desired frequency in less than 20 milliseconds. It is therefore very desirable to have a multifrequency monitoring arrangement that permits longer interruption periods so that frequency synthesizers can be used without causing noticeable distortion or deterioration in the quality of speech or information during channel switching.